Reference Designer Calculators

RC Time Constant Applications

RC time constant is one of the most widely used concept in engineering. We will present some of its applications

Reset Circuit Generator

A simple RC circuit can be used to generate power ON reset. It can replace a circuit like this one

With a simple circuit like this

The key is to keep the resistor R1 and the capacitor values high enough so that, the reset is generated long after the power and the crytals have stabilised. A tupical time constant of 500 ms is suffice for most POR curcuits.

Low pass filter

An RC circuit also acts as a low pass filter and can be used to filter out high frequency noise. Typical application includes Audio input circuit.

Consider a very fast rising edge incident to an RC circuit. The voltage at the capacitor takes some finite amount of time to rise. Since the waveform is rising very fast, it is able to reach only a very small fraction of its amplitude before the falling edge of the waveform arrives. In other words, the amplitude of the fast rising waveform is severly attenuated. A very slow rising waveform, on the other hand, rises to its peak as the RC circuit allows it because of its time constant.